1. Field of the Invention
This invention relates to a process for making polyvinylpyrrolidone (PVP), and, more particularly, to free radical solution polymerization of vinylpyrrolidone monomer in water in the presence of polyethylene glycol (PEG) as a chain transfer agent to provide an aqueous solution of PVP K-90 having a polydispersity value of less than 6, preferably less than 5, and most preferably less than 4, and PEG, in predetermined concentration.
2. Description of the Prior Art
During free radical solution polymerization of high molecular weight polymers at a high conversion, auto acceleration can occur, resulting in polymers with broad distributions and polydispersities up to 10. For many applications, broad distribution polymers are satisfactory. However, at equivalent molecular weights, narrow molecular weight distribution polymers have certain advantages over broad distribution polymers, such as higher bulk (Brookfield) solution viscosities. Also, the molecular weight and molecular weight distribution of certain polymers can affect their electron-beam resist sensitivity.
The art has employed costly equipment and special processes to make high molecular weight polymers with narrow molecular weight distribution. See U.S. Pat. No. 3,862,915.
The molecular weight distribution of high molecular weight polymers can also be controlled in a free radical polymerization at high conversion using chain transfer agents as additives. In such processes, chain transfer can occur in the free radical polymerization between the growing chain and the monomer, polymer and/or solvent. chain transfer agents have hydrogen atoms or other atoms which are more labile in a free radical polymerization than the hydrogen atoms on the monomer or polymer.
The chain transfer agent thus functions by terminating a growing chain by providing a more labile hydrogen atom to the growing chain. This mechanism can be used to control the molecular weight distribution of the polymer obtained. At high conversion, the viscosity of the solution increases, and auto acceleration can occur due to the reduction in chain termination processes from combination or disproportionation of the growing chains. Therefore, molecules terminated at high conversion will have higher molecular weights than molecules terminated at low conversion, yielding broader distribution polymers at high conversion. Chain transfer agents thus can provide an alternative termination process which is not expected to be hindered by viscosity built-up during polymerization, thus preventing the growth of high molecular weight molecules and reducing the breadth of the distribution.
U.S. Pat. No. 4,750,482 describes a hydrophilic, elastomeric, pressure-sensitive adhesive which is made from a solution of 20% PVP (K-90) powder, 25% PEG and 55% water. Accordingly, it is desired to form such solution during preparation of PVP polymer without requiring isolation of the PVP powder.
Accordingly, it is an object of this invention to provide a process for making PVP K-90 having a narrow molecular weight distribution, suitably a polydispersity of less than 6, preferably less than 5, and most preferably less than 4.
Another object herein is to provide a process for making such PVP K-90 polymer directly in the form of an aqueous solution of the 15-25% PVP, 15-30% PEG and 45-70% water.
These and other objects and features of the invention will be made apparent from the following description thereof.